The last century has seen great advances in the formulation aids and additives for cement, concrete and gypsum compositions. Such organic, inorganic, polymeric, and fibrous additives significantly broadened the scope of slurry preparation, speed of production, options in material processing and handling, and diversified means in precast and cast-in-place applications.
Advances in enhanced performance after curing covers early and final mechanical properties, speed of setting, water and chemical resistance, crack resistance, durability, and many other properties. However, one can still identify many areas where conventional technologies and processes are still used extensively today, yet lack the efficiency and capabilities to match the fast pace and new development in hydraulic materials, property enhancement additives, processes, energy requirements, desired specific performance, and cost parameters.
Even with the advancement in water reducing plasticizers, superplasticizers, polymeric binders, man-made mineral composites and fillers, as well as other setting and accelerating additives, ordinary Portland cement that consumes very high energy to produce and generates great amount of CO2 in the manufacturing process is still being used as the major hydraulically settable material for the building and construction industries. It is also well known that cementitious slurries typically need hours to set and days to produce adequate strengths for handling and use. Furthermore, for applications such as producing backer boards, blocks, panels, foundations, bridges, and roads that require proper strength development, surface finish, and absence of extensive shrinkage and cracking, proper curing conditions may often involve energy or labor-intensive and time-consuming steps.
On the other hand, hydraulic materials such as calcined gypsum, plaster of Paris, or stucco composites can set and develop strength quickly after adding water. Unlike Portland cement, the hardened gypsum products are often without shrinkage and produce good surface finish. Producing calcined gypsum (Calcium sulfate hemihydrate) from the gypsum mineral (Calcium sulfate dihydrate) requires much lower energy than that used in producing cement powder. However, in making gypsum slurries, a great amount of processing water is used (80-90% by weight of gypsum powder), and most of the water has to be evaporated in the drying process. The higher water solubility of hydrated gypsum products than that of hydrated cement makes most of these gypsum products unsuitable for exterior applications.
Numerous efforts were made to combine calcined gypsum with cement to achieve early strength development, great surface finish, and low shrinkage. However, most of such efforts failed—some catastrophically. Major causes of such failures are now well recognized as coming from the uncontrollable macrocrack formation when the hardened products were later exposed to temperature and humidity elements. Such late stage macrocrack formations are believed by many to have originated from the large ettringite crystals formed within the microvoids or microcracks that were present in the original hardened composites. Such secondary or delayed ettringite crystal formation—3CaO.Al2O3.(CaO.SO3)3.32H2O (DEF) is mostly derived from the interaction of some of the key components of cement (such as the tricalcium aluminate—3CaO.Al2O3) and the soluble gypsum species. After hardening and upon exposure to the environmental elements, the gypsum component, due to its higher solubility, may continue to react with tricalcium aluminate (3CaO.Al2O3) in the composite to form large crystals of ettringite (3CaO.Al2O3.(CaO.SO3)3.32H2O) and cause expansion and the associated macrocracking within the voids and crevices in the hardened composite.3CaO.Al2O3+3(CaSO4.2H2O)+26H2O=3CaO.Al2O3.(CaO.SO3)3.32H2O
To reduce DEF formation of such gypsum rich cement composites, U.S. Pat. Nos. 4,494,990, 4,661,159, and 5,958,131 describe the use of additives to react with the gypsum species. However, these prior art examples also recognize that such additives can only be effective in a very narrow range of weight percentages in a gypsum-cement mixture without causing significant adverse effects on the product. Moreover, the amount of gypsum used in the mixture is usually between 30 to 80% by weight, which is far greater than that typically used in a Portland cement formulation (4-6%). Prior inventions fail to clearly address the continuous and possible product deterioration upon long term humidity or water exposure at various elevated temperatures with the presence of such great amounts of the gypsum species (30-80%). The weight associated with the use of high amount of aggregates in the prior inventions also made the product much heavier than their cementitious or gypsum counterparts that were already being used in the industry. U.S. Pat. No. 6,197,107 addresses the post hardening expansion and macrocrack formation by DEF, but the invention is effective only in a very narrow range of the ratios among calcined gypsum, cement, and additives. The product was found to deteriorate rapidly and eventually totally disintegrate when the ratios fall outside of the narrow window. Such composition constraints need of excessive aggregates, and need of elaborate curing process are the reasons why using such additives was generally unsuccessful.
Anionic, cationic, zwitterionic or hydrolyzed protein based foam agents commonly used in making lightweight cement or gypsum products are known for their limited foam stability. Typically, the foam will start to show volume shrinkage or foam collapse as soon as they were made. The volume shrinkage or foam collapse of a freshly made foam is typically caused by the transformation of the lightweight foam back into its un-foamed liquid state.
Foam collapse typically comes from coarsening of the foam bubbles through the coalescing of small bubbles into larger ones. The large bubbles will burst easily and go back to their liquid state because of the fast liquid drainage from the bubble walls. The thinning or the liquid drainage of walls quickly destabilizes the bubbles and causes them to burst. Many attempts have been made to increase the foam stability and reduce liquid drainage. One such attempt is the use of an anionic foam stabilizer to stabilize a cationic foam (U.S. Pat. No. 5,696,174).
To reach a stable aqueous foam, U.S. Pat. No. 5,696,174 employs a two component approach using a foaming agent stabilized with a foam stabilizer. This is similar to the two components epoxy-amine system or the two component isocyanate-polyol system. However, unlike the latter two that use crosslinking chemistry to permanently lock the two components together, these two components turn the foam back to the initial un-foamed liquid state after most of the foam liquid is drained off from the foam surface and causes foam collapse.
Most of the prior arts dealing with foam stability in making lightweight cement or gypsum articles are focused on the efficiency of such foams in reducing the weight of the hydraulic article, being low cost and complying with the minimum compressive strength required before significant foam collapse takes place. In such industrial applications, very little information is available on the study of pore fineness, the means to prevent the foam structure from coarsening, and the benefit and justification of maintaining the stability of a fine-pored foam towards making of the foamed hydraulic articles. No prior art was found on how a very light and stable aqueous foam can be used as a dispersing media or carrier for fibrous, organic, inorganic, and polymeric additives used in a hydraulic slurry.
The present invention provides solutions to many of the problems set forth above. Additionally, the present invention provides a foam carrier that may carry accelerators, retarders, wax, polymer binder, and fiber or some other very reactive components for a typical slurry. Using such a foam carrier, a slurry itself may contain only water, water reducer, and hydraulic powder. This approach has the advantage of opening up all kinds of process possibilities and flexibilities, such as avoiding plant mixer downtime for making fast setting composites and making hydraulic articles with targeted performance features, which would be otherwise unattainable if such components are directly mixed into slurries.